This invention relates generally to direct-acting pressure sensors, and more particularly to a pressure gauge of the elastic type which is operable in the very low-pressure range.
Measurement of absolute pressure, gauge pressure, vacuum and draft pressures, and differential pressure, is carried out by two primary types of pressure-sensitive elements, the first being the liquid column in which the height and density of the liquid are utilized to measure pressure, and the second being the elastic pressure device. The sole concern of the present invention is with elastic pressure elements which are designed to follow the physical law that within the elastic limit, stress is proportional to strain; hence deflection is proportional to applied pressure.
The Bourdon tube, because of its stability, simplicity and high pointer torque, is widely used as a pressure or vacuum indicator or controller. The operation of the Bourden tube is based on the principle that an elastic tube having an internal cross-section that is not a perfect circle, if bent or distorted, has the property of changing its shape with internal pressure variations. This internal pressure causes the cross-sectional form to become more circular, giving rise to a motion of the closed end of the tube if the open end is rigidly fixed. This motion is called "tip travel."
The Bourden tube comes in three main types. The C-type is formed by winding the tube to define a segment of a circle, whereas the spiral type is created by winding more than one turn of the tube in the shape of a spiral about a common axis. The helical type is created by winding several turns of the tube in the shape of a helix. A Bourden spring in any of the existing types can be made from any metal or alloy which exhibits satisfactory elastic qualities.
While Bourden tubes are capable of operating within various pressure ranges running as high as 100,000 psi, in no instance is it possible as a practical matter, to operate such tubes below 12 or 15 psi. Hence, despite the advantages of Bourden tubes, they are not effective as gauges in the low-pressure range and it has heretofore been necessary to employ costly and relatively fragile bellows or diaphragm-sensing elements to effect measurement in this range.
My prior U.S. Pat. No. 3,732,733 (Reick and Schmaus) discloses a low-cost pressure sensor capable of accurately gauging pressures in the low pressure range. The sensor disclosed in this patent includes two curved flat metal springs in a concentric arrangement wherein the springs are cantilevered from a socket, the free end of the springs being joined together at a tip from which a pointer extends. The springs enclose a flexible bladder having an internal chamber which communicates with the socket, whereby fluid fed through the socket into this chamber acts to expand the bladder and causes the springs to uncurl, thereby moving the tip and the pointer attached thereto as a function of the applied pressure.
In a conventional Bourden tube pressure gauge, the relationship between applied fluid pressure and tip travel is such that it takes a large pressure change to effect a relatively small tip movement. Consequently, commercial forms of such gauges include gear works or linkages to mechanically amplify the tip travel. The friction introduced by mechanical amplifiers usually makes it necessary to tap the gauge to cause it to register. Such tapping is unnecessary with a sensor of the type disclosed in my prior patent, for no mechanical amplifier is involved. The sensor is highly sensitive so that a small change in applied pressure produces a relatively large tip excursion.
Moreover, with this patented sensor, since there are no gears or linkages and the tip is directly coupled to the pointer, there is no problem of wear and the sensor has a high shock resistance as well as a long life expectancy.
While a sensor of the type disclosed in my prior patent is capable of measuring fluid pressures below 12 or 15 psi in a range in which a conventional Bourdon tube is unresponsive, because the bladder is loaded by two curved, flat metal springs, my patented sensor is insensitive to very low pressures in the range below 350 mm Hg (one atmosphere = 760 mm Hg). Moreover, curved metal springs of the type disclosed in my prior patent are relatively difficult and costly to manufacture.
The need for sensors or gauges operating in the very low pressure range is widespread, and while such sensor exists, they are relatively complex and costly.